The social backdrop of a novel usually becomes apparent through historical events, fashions or figures of speech. Philip Hensher's can be dated by its canapés. The miniature quiches, mushroom cases and cheese-and-pineapple on sticks served up in the opening scene indicate that this has to be a provincial dinner party circa 1974. Unless, of course, it is actually 20 years later, and such things are considered ironic.

Hensher's epic novel is set in Sheffield and spans the rise, fall and return of the vol-au-vent as a social accessory. In the opening scene, timorous hostess Katherine Glover hands round plates of nibbles while her teenage son Daniel lolls on the sofa leering at the female guests, who are perturbed by his ill-concealed erection. By the end of the book it is the mid-1990s: Daniel has settled down and established a modish restaurant in a redundant forge, where the starters come speared to a foil-wrapped potato.

The book aspires to be a great deal more than a social history of the cocktail stick, however. It's clearly intended to be a state-of-the-nation account of the Thatcherite era from the perspective of one of Britain's most militant leftwing outposts. You might expect Hensher to know Sheffield well, as his parents moved there from south London when he was nine, and there's an apparently autobiographical thread in the story of the Sellers family, who pack up their London home and head north. The figure of teenage son Francis - diffident, studious and self-conscious about his height - is quintessentially Hensherian, while stroppy sister Sandra, who insists on riding up front so she can flirt with the removal men, is clearly going to be a handful.

The journey is marked by some slightly baffling outbreaks of purple prose: "London had gone on for ever, its red brick houses and businesses clinging to the edge of a motorway, like small rodents to a balloon suddenly in flight." Why, you wonder, would a string of field mammals be hanging on to a balloon in the first place? And what does Hensher mean exactly when he evokes Sheffield as a city of "dense Victorian villas dispersed through a verdant forest, breaking out like the frilled edges of amateur maternal pancakes into lavender moorland"? It seems peculiar to apply this particular culinary image to one of Britain's hilliest cities when pancakes are, in general metaphorical usage, flat.

Even the title itself is a puzzle. The dictionary defines clemency as a condition of mildness or mercy, neither of which seems particularly apt for Sheffield throughout the turbulence of the Thatcher years. Perhaps it's a reference to the temperate disposition of its chief protagonists, who are unassuming to the point of anonymity. Picturing the main characters is not helped by Hensher's parsimonious approach to physical description - it's almost impossible to put a face to Katherine, as we are nearly two-thirds of the way through the book before we learn she is a standard-issue Tory mum with the "little pie-crust collar Mrs Thatcher liked to wear on television". The only concrete thing we know about her husband Malcolm, who works in a building society, is that he's a member of a battle re-enactment society, so it may be helpful to picture him in Roundhead uniform. It is stated that their son Tim has a head "like a bowling ball sitting on his shoulders", though not until page 267.

The biggest surprise is that, among all these characters, weird Tim is the only figure remotely connected to the political context of the time. Traumatised during childhood when his mother treads on his snake, Tim develops into a disaffected, mean-spirited activist, poring over Marx in the public library and handing out socialist literature with his girlfriend, whose commitment to women's issues is signified by her pungent armpits. There's an unconvincing episode in which he bumps into Arthur Scargill outside the Orgreave coking plant. But for the rest of the characters, the miners' strike is only a vague presence, manifested by the presence of a few women in the city centre, collecting tins of food.

Late in the book Daniel meets Helen, the daughter of a striking miner; though when she takes him to meet her parents he is surprised to find the neat little estate where they live "wasn't at all like the towns of the poor he had always imagined, nothing like a place where people worried about food". Helen's exasperation with Daniel eventually boils over: "Sometimes I can't believe you were born in this town - you don't seem to understand anything about anything."

Perhaps this is Hensher's overall point - that throughout any period of social and political turmoil there will be a stratum of blithely unobservant middle-class people who can barely be bothered to switch on the news. It might explain why there's only one fleeting reference to David Blunkett, whose influence as council leader shaped the so-called People's Republic of South Yorkshire; though it's hard to countenance an epic novel of the 1980s that contains only one brief mention of the Falklands war and says nothing about South Yorkshire innovations such as cheap public transport, the introduction of peace studies in schools or the establishment of a nuclear-free zone. It's a book whose girth accrues not through the weight of history, but because the decision to make a fish pie invariably entails a two-page trip to the fishmonger. Perhaps, as with the retro offerings at Daniel's restaurant, we are supposed to appreciate the irony. But how many miniature quiches can you eat without coming to the conclusion that it's only so much stodge?